This is the fourth post in a series of five video blogs explaining how to measure mobile media.

The use of location to measure the impact of mobile media is mobile-specific and is used to help bring a smartphone user into a physical retail store.

Almost everyone has location tracking enabled on their smartphone. This is one of those pieces of data that ad networks collect every time they serve an ad to you. The ad network can select people who have come within a specific range of a retail store and serve up an enticing offer that gets them to respond by walking through the door.

You can use this method to measure the impact of your campaign by holding out a control group that is not served the ad and determining whether your ad had any effect on purchasing behavior for the group that received the ad in comparison. Remember, the ad network knows who received the ad and can show whether those people went into the store. The ad network can also show data on people who were in the targeted geographic area and who were in the target group but were not served the ad (as part of the control group). It can also show how many of them entered the store without having seen an ad. The relative difference is how you can tell whether your campaign was effective.